Clarify how we derive ed25519 for cross-certification.

The descriptor format uses a curve25519->ed25519 conversion
algorithm to cross-certify descriptors with their ntor onion keys.

This patch clarifies two aspects of the algorithm:

1. When deriving a private key, how to derive the part of the
   private key that _isn't_ a point on the curve.

2. That there are two algorithms here, one for private->private and
   one for public->public.

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